Re: [Discuss-gnuradio] Large RX Delay

[Eric Blossom]

On Mon, Nov 13, 2006 at 10:19:22PM -0800, Thomas Schmid wrote:
> 
> No, I do not log the received data into a file. I record the wave
> forms on the oscilloscope and do a post processing on them in octave.
> 
> >> First of all, I don't understand why we have such a high delay.
> >> Shouldn't it be more in the hundreds of /mu s instead of in the ms
> >> range?
> >
> >Yes, at the high input rates.
> >
> >> Second, why is the delay shorter for decimation 64, and again
> >> larger for a decimation of 256?
> >
> >Underruns and/or incorrect examination of the incoming data?
> 
> I think now, after considering your response, that this might be the
> problem. I will check both things tomorrow and do the measurements
> again. Hopefully I will find smaller delays.
> 
> >Also, the received data (and transmitted data too) is "quad buffered"
> >in the FX2, so there's a maximum of 4 512-byte buffers between you and
> >the data on the receive path.  This could be reduced without much
> >trouble to "double buffered".  But I don't think this is really the 
> >problem.
> >
> >With the quad buffered case and decim = 8, the most data that could be
> >buffered in the FX2 is 4*512 = 2048 --> 2048/32e6 = 64 us worth.
> >
> >Right now I'm looking at how the received data buffering is done in
> >the usrp and gr-usrp code.  Looks like there may be a
> >problem/opportunity in the usrp1_source_base.cc.
> >
> >I'll get back to you with more info in a bit...
> 
> Thank you very much. I am looking forward to your response.
> 
> Thomas

Thomas,

I've made a change to usrp1_source_base.cc that might help.
You can either pick it up from my developer branch:

  $ svn co http://gnuradio.org/svn/gnuradio/branches/developers/eb/rx-latency

or apply the attached patch.

Please let me know if it helps.  I think it will, once the overruns
are take care of.  If it does work, I'll merge it into the trunk.

Eric

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